1. Field of the Invention
The present invention relates to a printing apparatus, a resolution conversion printing method for the printing apparatus, an image processing method for the printing apparatus, and a storage medium storing a computer-readable program.
2. Related Background Art
Conventionally, in a printing apparatus such as a dot-matrix page printer, e.g., a laser beam printer, an inkjet printer or the like, vector data representing a figure, a character and the like is converted into raster data composed of sets of pixels (dots), and the obtained raster data is expanded or extracted in an internal memory space corresponding to one page. Based on the expanded data, toner or ink is adhered to a specific position (within a range corresponding to each pixel area) on a printing medium, thereby performing printing.
Hereinafter, such a printing mechanism of the laser beam printer will be explained by way of example.
Printing data transferred from a host computer is expanded by a printer controller into raster data composed of sets of pixel data. At this time, the number of pixels which compose the raster data and are assumed to be existent in unit area is called "data resolution".
Then a video signal is generated based on the raster data, and the obtained video signal is converted into a laser driving signal for controlling on and off of a laser beam. The laser beam generated based on this laser driving signal (i.e., the video signal) is irradiated onto a photosensitive drum previously electrified by negative charge.
When the laser beam scans the electrified photosensitive drum, the charge on the portion to which the beam was irradiated disappears, whereby a potential difference occurs between the portion to which the beam was irradiated and the portion to which the beam is not irradiated. Thus a latent image of which shape is identical with that represented by the raster data is formed on the photosensitive body. When toner of positive charge is adhered to the photosensitive drum on which the latent image has been formed, a visible image is formed.
In the printing apparatus having an above-described feature, there are two kinds of laser beam scanning directions, i.e., one is a horizontal direction which is parallel with a sheet carrying direction, and the other is a vertical direction which is perpendicular to the sheet carrying direction. First, along a line on the photosensitive drum, the laser beam scanning is performed in the direction parallel with a drum axial direction (main scanning). When the scanning of this line ends, the scanning is performed along a next line. On the other hand, since the photosensitive drum is rotatively driven, a start position of the main scanning is off to a direction perpendicular to the drum axial direction (sub scanning). Thus the latent image is formed in a two-dimensional area on the sheet.
At this time, the number of pixels capable of being represented in the unit area on the drum is called "engine resolution". When the main scanning speed is constant and the laser beam is on and off for each pixel, the engine resolution in the main scanning direction is determined according to an interval of on and off, the engine resolution in the sub scanning direction is determined according to rotation speed of the photosensitive drum.
Incidentally, the raster data generally has information of one bit or plural bits for each pixel.
Ordinarily, in a monochrome printing apparatus, one pixel often has information (binary information) of one bit (0 (representing "off") and 1 (representing "on")). In this case, a laser beam which is to be modulated based on such raster data is modulated only to be "on" or "off" for each pixel. Therefore, when printing data received from the host computer is multivalue data, the monochrome printing apparatus has to convert this multivalue data into binary raster data according to pulse-surface-area modulation (i.e., an area gradation method) or the like. In such the printing apparatus, printing quality tends to improve by increase of the resolution of the image to be printed.
Further, like many kinds of color printing apparatuses, when the raster data represents halftone for each pixel by plural-bit information, there are several methods of representing halftone density. For example, there is a method of changing an on time of the laser beam for each pixel according to density, a method of changing a charge level according to density, or the like. In any method, when the number of gradations of the printing data received from the host computer is different from the number of gradations representable by the printing apparatus, like the binary data, it is necessary to convert the number of gradations of the printing data received from the host computer into the number of gradations representable by the printing apparatus. In such the printing apparatus, printing quality improves by increase of the number of representable gradations as well as the resolution of the image to be printed.
In ordinary printing, optimum printing quality can be kept when the data is output with the engine resolution same as the data resolution.
As described above, when the laser beam is on and off for each pixel, it is necessary to increase both the data resolution and the engine resolution in order to improve the image quality.
Further, in the ordinary data, since the resolution in the main scanning direction is identical with that in the sub scanning direction, it is necessary to increase the data resolution and the engine resolution respectively in both the main and sub scanning directions.
At this time, since the engine resolution in the main scanning direction can be electrically changed by controlling an on/off interval of the laser beam, when the engine resolution in the main scanning direction is increased, an influence on engine speed is comparatively low. However, when the engine resolution in the sub scanning direction is increased, since a drum rotation angle is decreased, the engine speed (i.e., printing speed) is resultingly slowed down. Thus, there is a problem that the slowdown of the engine speed greatly influences a printing time.
Further, for example, when the engine speed (i.e., the printing speed) is set to be double speed, since the resolution in the main scanning direction is identical with that in the sub scanning direction as described above, it is necessary to print the data as setting not only the engine resolution in the sub scanning direction but also the engine resolution in the main scanning direction to be low. Thus, there is a problem of extremely lowering the printing quality.